1. Field of the Invention
The present invention relates to an antiskid control apparatus capable of preventing an inadequate operation such as wheel locking even when vibrations of a vehicle are taken place due to a bad road when the vehicle is braked.
2. Discussion of Background
FIG. 9 is a diagram showing an example of operation of a conventional antiskid control apparatus to prevent reduction in a brake pressure at a bad road at an early stage, disclosed in, for instance, Japanese Unexamined Patent Publication No. 261566/1987.
In the conventional antiskid control apparatus, as shown in FIG. 9, a provisional wheel speed 102 which traces an actual wheel speed 101 with a difference of speed which is a predetermined speed .DELTA.V.sub.0 lower than the wheel speed 101, is used. The provisional wheel speed 102 is so designed as to decrease with a constant gradient of speed reduction .theta. from the time when the deceleration of the wheel speed 101 has reached a predetermined value. When the provisional wheel speed 102 becomes equal to the actual wheel speed 101, the reduction of a brake pressure 103 is started.
Then, a time T from the time of starting braking or a high peak point of wheel speed to the next high peak point of wheel speed, a time t from the low peak point of wheel speed during the time T to the high peak point, and a difference of speed .DELTA.V between the low peak point to the high peak point are measured.
When the time T and the time t are respectively shorter than a predetermined time T.sub.0 and a time t.sub.0, and the difference of speed .DELTA.V is larger than a predetermined difference of speed .DELTA.V.sub.0, the predetermined difference of speed .DELTA.V.sub.0 determined for the provision wheel speed 102 is changed to a predetermined difference speed .DELTA.V.sub.1 which is larger than the difference of speed .DELTA.V.sub.0. Thus, the initiation of reducing the brake pressure 103 at an early stage can be prevented, and elongation of the braking distance is prevented.
In FIG. 9, a symbol P101 indicates the brake pressure in the first cycle and P102 indicates the brake pressure in the second cycle or subsequent cycles.
In the conventional antiskid control apparatus having the operations described above, when the brake is operated while the vehicle runs on a road having a low frictional coefficient, there is a possibility of vibrations in wheel speed due to a torsion in the driving system. When such vibrations in the wheel speed take place, a judgment of bad road is made and a correction to a bad road is effected to thereby prohibit the pressure reduction of the brake pressure; this may cause a danger of wheel locking.
Further, when an automobile runs on a bad road and vibrations take place in the wheel speed, the bad road has to be correctly detected in order to effect correction. Otherwise, the brake pressure is excessively reduced, whereby the brake distance is increased.